The present invention is especially useful in applications requiring a large active surface of the cell, such as displays, antiglare devices, welder's goggles etc. The background art and the advantages of the invention will therefore be described primarily with reference to displays, which thus should not be considered limitative of either the field of use of the invention or the inventive scope as defined in the appended claims.
Liquid crystal cells or displays of the type stated by way of introduction are well-known in the art. One or both plates are made of a transparent material, such as glass, and at least one of the electrode structures applied to the plates is also transparent. Moreover, there are traditionally orienting layers for aligning the molecules of the liquid crystal in the interface layer to the plates.
Under the influence of an applied electric field, the liquid crystal material can change its local orientation with respect to the direction of incident light, so as to affect the polarisation, absorption or scattering of transmitted or reflected light. By applying polarisers on one or both sides of the cell, the change of polarisation can be observed.
Normally, the optical effect is a function of the thickness of the liquid crystal layer, for which reason a local change of the thickness most often has a strong adverse effect on the performance of the display, which at worst may become completely useless due to variations of the thickness of the liquid crystal layer.
The most commonly used liquid crystal displays contain nematic or chiral nematic liquid crystals with positive dielectric anisotropy. In these displays, the optical axis of the liquid crystal material most often is parallel to the plates and, if so desired, twisted through the layer, most commonly through an angle of 90.degree.. In this so-called waveguide mode, the polarisation of the incident light coincides or essentially coincides with one of the local elgenmodes of the system. When applying an electric field over the liquid crystal layer, the optic axis of the material reorients along the field with the result that the polarisation of the light is not affected by the liquid crystal.
In addition, liquid crystal displays can operate with a variety of other electro-optic effects which are not based on the waveguide mode and where the modulation of light is instead due to more general changes of the polarisation of the incident light. Displays whose optical properties are based on such effects are generally highly sensitive to thickness variations of the liquid crystal layer and, consequently, to changes in the mutual spacing of the plates.
The thickness of the liquid crystal layer generally affects not only the colour but also the voltage-contrast dependence of the cell, the maximum accessible contrast and other parameters.
As described in U.S. Pat. No. 4,653,865, it is of particular importance to maintain a constant liquid crystal layer thickness in thin TN displays (Twisted Nematic) and STN displays (Super Twisted Nematic), and especially when the twist angle in the latter case amounts to 270.degree. or more. A constant thickness of the liquid crystal layer has become an increasingly important requirement for TN and STN displays, since to increase the switching speed, there is a trend towards ever thinner liquid crystal layers, even below 4 .mu.m. Such displays do not operate in the waveguide mode, and the optical transmission is highly sensitive to small thickness variations.
Another aspect of the problem is how to maintain the plates parallel over the entire display surface, and how to cope with the problem of display rigidity. Changes in the plate spacing due to mechanical stresses and/or temperature may lead to macroscopic flow of the liquid crystal material, which in turn may damage the orientational layers on the inside of the plates. A reduction of the plate spacing may even entail local short-circuiting of the display, and the risk of this occurring increases as the initial plate spacing of the liquid crystal display decreases.
It is true that a TN cell may often return to its normal optical state after an applied external pressure, such as a thumb pressure, has been removed. Normally, this also applies to an STN display, provided the twist angle is not too large, say for twist angles of 180.degree.-200.degree., but not for larger twist angles, for example of 270.degree., where the pressure deformation causes a change in the helix structure which may easily become irreversible.
At best, realignment may then be achieved by heating and subsequent cooling of the display and/or by applying an electric field.
To conclude, an STN display having a high twist angle may thus be easily damaged if subjected to external pressure.
Damage caused by mechanical pressure on smectic displays containing ferroelectric (FLC) or antiferroelectric (AFC) liquid crystals is a major problem today as discussed e.g. in a review by S. T. Lagerwall, N. A. Clark, J. Dijon and J. F. Clerc, Ferroelectrics, Vol. 94, 3, 1989. Irrespective of the nature of the specific smectic material, all smectic displays where the layers are not parallel to the cell plates, are extremely sensitive to plate deformations and, hence, are sensitive to shocks.
Smectic displays use a completely different mechanism which limits the permissible deformation as compared with the TN and STN cases. Generally, smectic displays are considerably more sensitive to shocks than corresponding nematic ones. In a first approximation, the layers are standing perpendicular to the glass plates (ideal upright bookshelf geometry), but in a more detailed model, the layers are most often angled with respect to the glass plates in a so-called chevron structure. The structure is known as a QBS structure (Quasi Bookshelf Structure) when the angle of the layers to the normal of the plates is very small. Both the chevron and the QBS structure are extremely sensitive to pressure, and for both FLC and AFLC, the order of the layer structure is damaged by direct application of thumb pressure.
Normally, a chevron structure cannot be realigned without heating, whereas a QBS structure not excessively deformed can be realigned by application of a moderate AC field. Beyond a certain deformation limit, the layer order however becomes irreversibly ruined, as in the chevron case. The display then becomes unusable.
To achieve a liquid crystal cell having a well-defined layer thickness over a relatively large surface, it is known from U.S. Pat. No. 4,150,878 (filed in the name of Barzilai et al. in 1978) to use prestressed 3-mm glass plates with a plurality of spacers or supporting points distributed in the cavity. The function of the spacers primarily is to provide a uniform layer thickness over the entire surface, i.e. good surface parallelism. In a first example, use is made of cylindrical spacers having a diameter of 50 .mu.m distributed with a mutual spacing of 0 mm, which is said to result in a surface parallelism with a 0.1 .mu.m-0.6 .mu.m tolerance over a surface of 10 cm.times.10 cm. In a second example in the same document, use is made of spacers having a diameter of 0.1 mm with a mutual spacing of 1 mm, which is said to give a surface parallelism with a 0.2 .mu.m tolerance. These tolerances are however insufficient and unacceptable for today's liquid crystal displays (neither STN nor FLC existed in 1978). Moreover, the spacers of this known screen do not result, as will be explained in more detail below, in a pressure-insensitive display. Furthermore, both the 3-mm plate thickness and the spacer diameters are unacceptable in commercial contexts.
EP-A2-0 407 993 (priority year 1989, and applicant Hoechst AG) describes a method for making an LCD cell. To achieve high uniformity of layer thickness, uniformly distributed spacers are etched in one glass plate. This document does not contain any indication about the deformation properties of the cell, in addition to an unconfirmed statement that the cell is "mechanisch sehr stabil". The cell is however described in Rieger et al. Proc., SID Conference, 6-10 May 1991, Anaheim, USA. Since it has also been shown at different conferences, its mechanical properties are well-known. From the last-mentioned article and from public demonstrations, it appears that the cell is easily deformed by thumb pressure. From both the EP document and said article appears that the plate thickness is 0.5 mm, the spacers having a size of about 20 .mu.m and being distributed with a spacing of 600 .mu.m.
The displays hitherto manufactured and demonstrated thus are not mechanically rigid or shock-resistant in that a mechanical pressure, for example thumb pressure, applied locally on the protective glass deforms this and causes both optical changes and flow of material in the cell. This is a general problem in liquid crystal cells, and especially in smectic liquid crystal cells, whose development towards technologically high-potential FLC and AFLC displays has been hampered thereby.
The most direct approach towards obtaining a pressure-insensitive display--using thick plates, for example in the order of 3 mm or more--is unrealistic for today's thin LCD cells. The maximum thickness used today normally is 1 mm, and thicker plates have completely disappeared in practice. Currently, there are also thicknesses of 0.9 mm, 0.6 mm, 0.5 mm and 0.3 mm, of which the latter dimensions have hitherto been used only for very small displays.
The object of the present invention is to obviate these problems encountered in currently used liquid crystal cells, especially cells having a relatively large active surface.
A main object of the invention thus is to provide a liquid crystal cell of the type stated by way of introduction, which is essentially insensitive to normally occurring mechanical influence, for example during handling and transportation.
A special object of the invention is to provide a pressure-insensitive cell that is usable for both nematic (TN/ST) and smectic (FLC/AFLC) liquid crystals.
Another object of the invention is to provide a pressure-insensitive cell which also can be manufactured by a reliable and accurate method.
Yet another object of the invention is to provide a pressure-insensitive cell which can be implemented as a display having at least VGA resolution.
A further object of the invention is to provide a pressure-insensitive cell which can be implemented as an antiglare device, a welding shield and the like.
These and other objects and advantages are achieved by means of a liquid crystal cell having the features stated in the independent claims, preferred embodiments being recited in the dependent claims.